/**
 * @author zz85 / https://github.com/zz85
 *
 * Based on "A Practical Analytic Model for Daylight"
 * aka The Preetham Model, the de facto standard analytic skydome model
 * http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf
 *
 * First implemented by Simon Wallner
 * http://www.simonwallner.at/projects/atmospheric-scattering
 *
 * Improved by Martin Upitis
 * http://blenderartists.org/forum/showthread.php?245954-preethams-sky-impementation-HDR
 *
 * Three.js integration by zz85 http://twitter.com/blurspline
*/

THREE.ShaderLib[ 'sky' ] = {

	uniforms: {

		luminance: { value: 1 },
		turbidity: { value: 2 },
		rayleigh: { value: 1 },
		mieCoefficient: { value: 0.005 },
		mieDirectionalG: { value: 0.8 },
		sunPosition: { value: new THREE.Vector3() }

	},

	vertexShader: [

		"uniform vec3 sunPosition;",
		"uniform float rayleigh;",
		"uniform float turbidity;",
		"uniform float mieCoefficient;",

		"varying vec3 vWorldPosition;",
		"varying vec3 vSunDirection;",
		"varying float vSunfade;",
		"varying vec3 vBetaR;",
		"varying vec3 vBetaM;",
		"varying float vSunE;",

		"const vec3 up = vec3( 0.0, 1.0, 0.0 );",

		// constants for atmospheric scattering
		"const float e = 2.71828182845904523536028747135266249775724709369995957;",
		"const float pi = 3.141592653589793238462643383279502884197169;",

		// wavelength of used primaries, according to preetham
		"const vec3 lambda = vec3( 680E-9, 550E-9, 450E-9 );",
		// this pre-calcuation replaces older TotalRayleigh(vec3 lambda) function:
		// (8.0 * pow(pi, 3.0) * pow(pow(n, 2.0) - 1.0, 2.0) * (6.0 + 3.0 * pn)) / (3.0 * N * pow(lambda, vec3(4.0)) * (6.0 - 7.0 * pn))
		"const vec3 totalRayleigh = vec3( 5.804542996261093E-6, 1.3562911419845635E-5, 3.0265902468824876E-5 );",
		
		// mie stuff
		// K coefficient for the primaries
		"const float v = 4.0;",
		"const vec3 K = vec3( 0.686, 0.678, 0.666 );",
		// MieConst = pi * pow( ( 2.0 * pi ) / lambda, vec3( v - 2.0 ) ) * K
		"const vec3 MieConst = vec3( 1.8399918514433978E14, 2.7798023919660528E14, 4.0790479543861094E14 );",

		// earth shadow hack
		// cutoffAngle = pi / 1.95;
		"const float cutoffAngle = 1.6110731556870734;",
		"const float steepness = 1.5;",
		"const float EE = 1000.0;",

		"float sunIntensity( float zenithAngleCos )",
		"{",
			"zenithAngleCos = clamp( zenithAngleCos, -1.0, 1.0 );",
			"return EE * max( 0.0, 1.0 - pow( e, -( ( cutoffAngle - acos( zenithAngleCos ) ) / steepness ) ) );",
		"}",

		"vec3 totalMie( float T )",
		"{",
			"float c = ( 0.2 * T ) * 10E-18;",
			"return 0.434 * c * MieConst;",
		"}",

		"void main() {",

			"vec4 worldPosition = modelMatrix * vec4( position, 1.0 );",
			"vWorldPosition = worldPosition.xyz;",

			"gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

			"vSunDirection = normalize( sunPosition );",

			"vSunE = sunIntensity( dot( vSunDirection, up ) );",

			"vSunfade = 1.0 - clamp( 1.0 - exp( ( sunPosition.y / 450000.0 ) ), 0.0, 1.0 );",

			"float rayleighCoefficient = rayleigh - ( 1.0 * ( 1.0 - vSunfade ) );",

			// extinction (absorbtion + out scattering)
			// rayleigh coefficients
			"vBetaR = totalRayleigh * rayleighCoefficient;",

			// mie coefficients
			"vBetaM = totalMie( turbidity ) * mieCoefficient;",

		"}"

	].join( "\n" ),

	fragmentShader: [

		"varying vec3 vWorldPosition;",
		"varying vec3 vSunDirection;",
		"varying float vSunfade;",
		"varying vec3 vBetaR;",
		"varying vec3 vBetaM;",
		"varying float vSunE;",

		"uniform float luminance;",
		"uniform float mieDirectionalG;",

		"const vec3 cameraPos = vec3( 0.0, 0.0, 0.0 );",

		// constants for atmospheric scattering
		"const float pi = 3.141592653589793238462643383279502884197169;",

		"const float n = 1.0003;", // refractive index of air
		"const float N = 2.545E25;", // number of molecules per unit volume for air at
									// 288.15K and 1013mb (sea level -45 celsius)

		// optical length at zenith for molecules
		"const float rayleighZenithLength = 8.4E3;",
		"const float mieZenithLength = 1.25E3;",
		"const vec3 up = vec3( 0.0, 1.0, 0.0 );",
		// 66 arc seconds -> degrees, and the cosine of that
		"const float sunAngularDiameterCos = 0.999956676946448443553574619906976478926848692873900859324;",

		// 3.0 / ( 16.0 * pi )
		"const float THREE_OVER_SIXTEENPI = 0.05968310365946075;",
		// 1.0 / ( 4.0 * pi )
		"const float ONE_OVER_FOURPI = 0.07957747154594767;",

		"float rayleighPhase( float cosTheta )",
		"{",
			"return THREE_OVER_SIXTEENPI * ( 1.0 + pow( cosTheta, 2.0 ) );",
		"}",

		"float hgPhase( float cosTheta, float g )",
		"{",
			"float g2 = pow( g, 2.0 );",
			"float inverse = 1.0 / pow( 1.0 - 2.0 * g * cosTheta + g2, 1.5 );",
			"return ONE_OVER_FOURPI * ( ( 1.0 - g2 ) * inverse );",
		"}",

		// Filmic ToneMapping http://filmicgames.com/archives/75
		"const float A = 0.15;",
		"const float B = 0.50;",
		"const float C = 0.10;",
		"const float D = 0.20;",
		"const float E = 0.02;",
		"const float F = 0.30;",

		"const float whiteScale = 1.0748724675633854;", // 1.0 / Uncharted2Tonemap(1000.0)

		"vec3 Uncharted2Tonemap( vec3 x )",
		"{",
			"return ( ( x * ( A * x + C * B ) + D * E ) / ( x * ( A * x + B ) + D * F ) ) - E / F;",
		"}",


		"void main() ",
		"{",
			// optical length
			// cutoff angle at 90 to avoid singularity in next formula.
			"float zenithAngle = acos( max( 0.0, dot( up, normalize( vWorldPosition - cameraPos ) ) ) );",
			"float inverse = 1.0 / ( cos( zenithAngle ) + 0.15 * pow( 93.885 - ( ( zenithAngle * 180.0 ) / pi ), -1.253 ) );",
			"float sR = rayleighZenithLength * inverse;",
			"float sM = mieZenithLength * inverse;",

			// combined extinction factor
			"vec3 Fex = exp( -( vBetaR * sR + vBetaM * sM ) );",

			// in scattering
			"float cosTheta = dot( normalize( vWorldPosition - cameraPos ), vSunDirection );",

			"float rPhase = rayleighPhase( cosTheta * 0.5 + 0.5 );",
			"vec3 betaRTheta = vBetaR * rPhase;",

			"float mPhase = hgPhase( cosTheta, mieDirectionalG );",
			"vec3 betaMTheta = vBetaM * mPhase;",

			"vec3 Lin = pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * ( 1.0 - Fex ), vec3( 1.5 ) );",
			"Lin *= mix( vec3( 1.0 ), pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * Fex, vec3( 1.0 / 2.0 ) ), clamp( pow( 1.0 - dot( up, vSunDirection ), 5.0 ), 0.0, 1.0 ) );",

			//nightsky
			"vec3 direction = normalize( vWorldPosition - cameraPos );",
			"float theta = acos( direction.y ); // elevation --> y-axis, [-pi/2, pi/2]",
			"float phi = atan( direction.z, direction.x ); // azimuth --> x-axis [-pi/2, pi/2]",
			"vec2 uv = vec2( phi, theta ) / vec2( 2.0 * pi, pi ) + vec2( 0.5, 0.0 );",
			"vec3 L0 = vec3( 0.1 ) * Fex;",

			// composition + solar disc
			"float sundisk = smoothstep( sunAngularDiameterCos, sunAngularDiameterCos + 0.00002, cosTheta );",
			"L0 += ( vSunE * 19000.0 * Fex ) * sundisk;",

			"vec3 texColor = ( Lin + L0 ) * 0.04 + vec3( 0.0, 0.0003, 0.00075 );",

			"vec3 curr = Uncharted2Tonemap( ( log2( 2.0 / pow( luminance, 4.0 ) ) ) * texColor );",
			"vec3 color = curr * whiteScale;",

			"vec3 retColor = pow( color, vec3( 1.0 / ( 1.2 + ( 1.2 * vSunfade ) ) ) );",

			"gl_FragColor.rgb = retColor;",

			"gl_FragColor.a = 1.0;",
		"}"

	].join( "\n" )

};

THREE.Sky = function () {

	var skyShader = THREE.ShaderLib[ "sky" ];
	var skyUniforms = THREE.UniformsUtils.clone( skyShader.uniforms );

	var skyMat = new THREE.ShaderMaterial( {
		fragmentShader: skyShader.fragmentShader,
		vertexShader: skyShader.vertexShader,
		uniforms: skyUniforms,
		side: THREE.BackSide
	} );

	var skyGeo = new THREE.SphereBufferGeometry( 450000, 32, 15 );
	var skyMesh = new THREE.Mesh( skyGeo, skyMat );

	// Expose variables
	this.mesh = skyMesh;
	this.uniforms = skyUniforms;

};
